Check inductor resistance?

Discussion in 'General Electronics Chat' started by shortbus, Feb 20, 2015.

  1. shortbus

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    For my ongoing project, I went through my electronics junk and found some inductors. They are unmarked unidentified, naturally. When trying to measure the resistance of them, to try to get an idea of the amperage they are capable of, a snag happened. I've never used inductors before so maybe it is normal, couldn't find any information on it though.

    The inductors are heavy duty, out of junk industrial controls of some type. Toroidal, around 1 1/4" diameter, 3/4" wide, wound with 18Ga wire double layer. Figured they would be an ohm or two resistance. But when checked they only show 0.1 ohms? Using my Simpson 467 DVM. Does this sound right? Don't have a LCR meter, so what am I doing wrong here?
     
  2. DC_Kid

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    Feb 25, 2008
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    The "R" in LCR is typically "Z" (impedance), a 1kHz frequency and the meter measures current, coverts that to a Ohmic value ("Z") when in "R" mode. straight DC test from a std ohm meter, 0.1 ohm sounds about right. the "L" and "C" in LCR use the same method as the "R" setting, but the meter will convert that info over to L or C values for the display, etc.

    here's my LCR on a 1.5k resistor set to "R" mode, used scope to show you what the LCR is doing, etc.
    [​IMG]
     
    Last edited: Feb 20, 2015
  3. alfacliff

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    Dec 13, 2013
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    have you touched the leads of your meter together? most meters do not go all the way to zero when the leads are together due to lead resistance, circuit design and such.
     
  4. ronv

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    I'm surprised they are not lower. Hard to tell much about the inductance with a resistance reading.
     
  5. DC_Kid

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    lets be clear, "resistance" has many meanings. in general, there is DC Resistance, and then there's impedance, aka "Z", which is measured under std AC sine wave frequency of 1kHz. this "Z" value is a vector of raw DC-ohms + L + C, (L & C are opposites of each other, etc)

    so, knowing the DC-R value is one metric of the vector that forms "Z". L and C measurements using basic LCR meter is only good for L's and C's that have little to no R, etc.

    imagine having a L that had say 1million DC ohms. this technically will throw off actual L calculation because most basic LCR meters dont do a DC-R test 1st. a large L that has small wire where DC-R is big usually means the raw L value is also large, but basic LCR meter will use "Z" value in it's conversion over to L or C value.

    clear as mud, huh.......
     
  6. MaxHeadRoom

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    And basically the ideal L would have zero resistance, or as low as possible, resistance appears in series with the inductive reactance so the lowest R value is desirable.
    Max.
     
  7. #12

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  8. shortbus

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    Thanks guy's. To quote DC Kid, "clear as mud". :) First I assumed all DVM's showed 0.1 when the probes were shorted, at least mine do. But I was expecting a reading, similar to what a piece of wire "X" feet long would be. Why is there no ohm reading? Must not be asking Google the right question, again.

    While I didn't know there would be no resistance shown, it was mostly to make sure they had continuity. Which they do using the continuity checker on my meter.
     
    Last edited: Feb 20, 2015
  9. #12

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    Little tiny ohms are difficult to read with most meters. You're lucky if you can get the test leads connected well enough to show less than 0.1 ohm...which you already found out. That's why there is a market for low ohm testers. Send tracecom a friendly PM and he'll probably give you one.

    Anyway, no surprise that 15.66 feet of 18 gauge wire are required to produce 0.1 ohm. The current limit on those toroids is controlled by the magnetic saturation of the core. You just need to adjust your expectations.
     
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  10. shortbus

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    New question.:) I found a couple of boost convertor calculators on line. And since my application for the inductor is somewhat like a boost convertor without regulation, figured I could use them. But for my application the output voltage stays the same ~300VDC, but the switching frequency will vary for different types of uses, the inductor values needed also vary.

    Then I look at Data Sheets for an inductor, new problem. The DS gives two values for an inductor. The first is for 1kHz, the second is for the inductor at rated amperage. And neither are the same inductance. This is getting curiouser and curiouser!
     
  11. shortbus

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    Thanks for that #12. I just figured it would be like measuring the resistance of a transformer coil, solenoid or even a motor field coil. Still can't figure why an inductor is different, just that it is. :(
     
  12. #12

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    All those other things you mentioned are about using up power. Inductors generally aren't. They can be as low as milli-Henries at the speeds of modern switchers. Typical numbers might be 170 VDC with a core that saturates at 100 amp turns and 200KHz. Twenty turns will saturate the core at 5 amps and the milliHenries depend on the core material. At 200KHz and a 50% duty cycle, there is only 2.5 microseconds for the current to increase through the inductor. If you can get close to 5 amps flowing in 2.5 uSec It's no surprise that the fuse melts when a transistor shorts. :p
     
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  13. shortbus

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    I was getting 750mH and 1350mH for frequency of 25kHz and 50kHz. 100VDC to 300VDC at .5Amp. This is to be a non-retriggerable single shot voltage at these values. To give a voltage 'spike' to start my EDM sparks, can't find the thread to link it.
     
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